ABSTRACT. Metacystis truncata, a prostomatid loricate ciliate, was found attached to Thalassia tesudinum, a dominant seagrass of the coral reef lagoons of Veracruz, Mexico. the morphology of the ciliate and its lorica were studied in living and stained specimens and in those prepared for scanning electron microscopy. the lorica is cylindrical (64.6‐115μm) with a well‐developed anterior neck and a sac reinforced by 15‐26 rings. Cell body size is variable (19‐91 μm live), from ovoid to elongate: the terminal protruding vacuole is conspicuous in small and medium‐sized individuals; the macronucleus is terminal in the elongate forms, or located near the mid‐body in the ovoid ones. This marine prostomatid colonizes both natural and artificial substrates such as narrow polyethylene strips placed in an aquarium. the highest density was found in the summer. the present study represents the first description of the lorica of M. truncata and the first record on T. testudinum shoots.

A new epibiontic ciliate of the genus Metacystis is described on the seagrass Thalassia testudinum of the coral reef lagoons of Veracruz, Mexico. The ciliate was studied in living and stained specimens and under the scanning electron microscope. The cell body (10-35 x 10-18 microm in vivo) is transversely annulated (4-6 rings). The somatic ciliature consists of 22-30 longitudinal kineties, and patterned as 5-7 transverse kineties. The circumoral kinety is composed of kinetosomes closely spaced. The macronucleus diameter measures about 3-7 microm. The lorica (18-61 x 11-26 microm) has the posterior end round to conical or irregular with mucoid filaments. This prostomatid colonizes both natural and artificial substrates placed in an aquarium. Metacystis borrori n. sp. is a species that forms part of the ciliate community on Thalassia testudinum with a temperature range of 21-26 degrees C and a salinity of 32-40 per thousand.